Mice are communal beasts, just like rats. They live in groups, and separating them – as required for measuring food intake / food uptake or energy expenditure – stresses them, elevating cortisol levels and leading to to a host of unwanted side-effects. Using the right technology, however, obtaining separate food uptake recordings from communally housed mice is straightforward. This short article demonstrates just such an application, combining a Promethion mass measurement module (2 mg resolution) with RFID.
To identify an animal using RFID, a simple and quick injection of a subdermal PIT (Passive Integrated Transponder) tag, about the size of a grain of rice, is required. There are two broad types of PIT tags; half duplex (HDX) and full duplex (FDX). For a variety of reasons, HDX PIT tags are preferable. Any vet or trained animal care technician can insert the tag. (A number of people are experimenting with them too.)
The principle of HDX RFID PIT tags is simple. A nearby coil periodically generates an EMF field at (typically) 134 kHz. The PIT tag contains a resonant circuit that charges a capacitor while the coil is generating its EMF field. Then the coil switches from transmitting to receiving mode; the PIT tag uses the energy stored in the capacitor to generate its own EMF field, which transmits a unique ID code back to the coil. And voilà, RFID! (Super-over-simplified, you understand.)
So, we can separate individual mice easily. But what about food uptake? Well, Promethion has a unique mass sensor based, like a lab balance, on a load cell, that allows extremely precise food uptake measurements. You can read about the principle here.
A little simple design work and a short spell with a lasercutter resulted in a box that held the mass sensor and food hopper, and restricted access to the food hopper to one mouse at a time via a tube just wide enough for a single mouse to enter. The tube that limited access was adjacent to an RFID reader of our own design. (Existing commercial RFID readers are limiting and cumbersome; I frown on them.)
The graph to the left is worth a thousand words. Click to embiggen it. The red trace corresponds to the ID of the mouse; either absent (no mouse in the feeder) or at two different levels, one corresponding to the ID of one mouse, the other to the ID of her nestmate. The blue trace corresponds to the mass of the hopper, which clearly shows the disturbance caused by feeding, and the change in food hopper mass before and after each feeding event by each mouse.
As you can see, separating food uptake / intake data for each mouse is easy. The precise uptake amount of food consumed during each feeding event is easily and automatically obtained, together with meal duration and unique-to-Promethion data such as the force that the mouse applied to the hopper during the feeding event. Using this RFID-based technique, subtle differences between mice can be teased apart from the stress of isolation. Better for the mouse, and thus – because an unstressed mouse is a better experimental subject – better for research too.
What about separating the metabolic rates (energy expenditures) of individual mice in a communal setting? All I can say is, stay tuned.